The present invention relates generally to high-frequency power conversion, and more particularly to a base drive circuit used to turn on and turn off high-power switching transistors.
In the design of switch mode power supplies, the object is to produce an AC-type waveform which will then be transformed, rectified and filtered to produce a DC voltage. To generate the AC-type waveform, switch mode power supplies often utilize one, two, or more power switches in various configurations. Symmetrical configurations, those having an even number of power switches, generate AC waveforms by operating the switches so that when one-half of the switches are turned on the other one-half of them are off; one-half of the switches controls the application of a positive DC voltage to a load, while the other one-half controls the application of a negative DC voltage, with the result that by operating the switches in a push-pull manner an AC-type voltage is applied to the load.
Bipolar transistors are frequently used as the power switches because of their fast switching speed and ability to handle high voltages and currents. In order to rapidly turn on and off conduction of these transistors, and thus be able to generate a high-frequency AC-type voltage, a base drive circuit couples control pulses to the respective bases of the power transistors to switch them into and out of their conductive states.
Switching circuits of the type to which this present invention are directed typically include an input stage that couples a control signal, usually in the form of a pulse train, to the primary winding of an isolation transformer. The secondary winding is coupled to the base lead of the switching power transistor. The switching transistor drives a primary winding of a high-frequency power transformer.
In order to achieve high efficiency, the power transistor is operated at relatively high frequency (in the kilohertz range), requiring the power transistor to be driven into its conducting and non-conducting states quickly. To hasten transition of the power transistor from its non-conducting to its conducting state (i.e., OFF to ON), the amplitude of the forward base drive current (IB1) must be relatively high. Unfortunately, if the base drive current is maintained at this amplitude for the time of the pulse the base drive circuit consumes more power than desired, and the design of the base drive circuit must take this fact into consideration. In addition, the continuous high amplitude drives the transistor deeply into saturation, causing the turn-off time to be extended because of the time required to sweep out stored charge from the transistor before it can turn OFF, and resulting in excessive power dissipation in the transistor.
Further, in order to drive the switching transistor OFF in as short a transition time as possible, a reverse base drive current (IB2) having a negative-going amplitude can be provided. Unfortunately, to provide such negative-going IB2 signals, a negative power supply is often needed.